Microwave ablation of tissue requires electromagnetic energy at microwave frequencies to be delivered to a target site via a cable used as a conduit to contain the energy between the inner and outer electrical conductors in a coaxial arrangement. There are some limitations with using coaxial cables for this type of energy delivery. The power handling of microwave cables is related to a number of factors such as frequency of operation, cable diameter, and dielectric filling. The dielectric filling of the cable possesses a loss property which absorbs energy creating heat. The ratio of inner to outer conductor surface area also affects this loss property by focusing the power transported by the dielectric.
Typically, thin microwave cables have higher loss and cannot accommodate power compared to larger diameter cables. In turn larger cables are more rigid and feel restrictive for the user. In medical applications dexterity is an important human factor in surgical treatments and it is desirable for medical devices not to significantly impinge upon the user's freedom.
In applications where energy is reflected by the termination, for example in medical ablations, this type of cable heating problem is compounded as the returning reflected energy is absorbed and dissipated as heat by the cable. In addition this return energy is superimposed onto the delivered energy as a result of voltage standing wave (VSW) creating localised excessive heating (hotspots) within the cable at fixed points. This can be particularly problematic in medical applications where stringent regulations govern the temperature of patient and user contacting parts to prevent inadvertent burns from cabling.
Additionally this phenomenon can shorten the lifetime of cables by burning the dielectric at the hotspot location by creating absorbing regions that increase the attenuation within the cable.
One method to overcome the issues with cable heating is to use a thin cable with a circulating cooling fluid jacket. The result of this approach is a flexible cooled cable however it can be easily damaged and has lower power handling performance coupled with complex waterproof encapsulation which has the possibility to leak resulting in expense to manufacture and reliability issues. Other methods include covering the cable with extra insulation layers which tend to increase the rigidity and traps the heat or placing the cable through a folded support platform (cardboard or plastic) to separate the cable from the patient.
Another aspect of design in medical applications is unwanted electromagnetic radiation emission. In medical microwave applications unwanted radiation is often not necessarily at the frequency of the treatment (for example 1-10 GHz) and may occur at other radio frequencies such as for example in the 5-200 MHz ranges causing electromagnetic interference (EMI) to nearby equipment. There are medical device and FCC requirements and standards set to limit this type of non-intended radiation which pose a challenge to system designers. Problems may arise when the connecting cabling is electrically isolated from the system ground or “floating”. One issue with this approach is that the cable is at a different electrical potential to the system ground such as in Type B floating medical devices (Type BF). Spurious emissions from internal circuitry and internal wiring that are normally contained with the enclosure induce currents on the floating components. Any cable connected to the floating parts carries off these currents and acts as an antenna as it emerges from the system ground plane creating the unwanted radiation. Some techniques involve connecting the outer of the microwave coaxial cable to the zero volt side of an isolated power supply which may also include bypass capacitor(s) to couple high frequency noise to the system ground.
Microwave cables are typically manufactured using industrial microwave techniques with connectors attached to the outer and inner conductors of the coaxial cable. The connectors are then fastened to a port and typically locked into place. As they are affixed at one side these type of cables possess a torsional rigidity and hence lack fluidity during use, in some instances they will tend to coil or will resist being straightened. This becomes more pronounced with larger cables which also have increased weight and limits the freedom of the end user.
In many treatments the cable and applicator are integrated and after use the entire assembly is disposed leading to a significant additional expense for the procedure. Microwave cables are typically very expensive due to the materials and manufacturing tolerances required to achieve microwave performance. This expense tends to increase with the operational frequency and loss/performance specification of the cable. One option is to retain the majority of the cable between treatments and use a short interconnected disposable applicator/cable portion for the patient. The benefits of this are that the long cable can be low loss high specification to maximise the energy delivery with the disposable portion being low cost to reduce the manufacturing and subsequent treatment costs. This approach is however limited due to the fragility of the cable as the coaxial structure is particularly sensitive to damage especially at microwave frequencies.
Cables that are crushed or excessively bent may change the coaxial ratio causing them to reflect or absorb energy resulting in poor performance.
There is therefore a need for a method and device for the delivery of microwave energy, for example in medical environments, that protects the patient and/or user from unwanted heat, is pliable by the user and offers long term mechanical protection of the cable whilst preventing unwanted electromagnetic radiation.